This invention relates generally to toners, and particularly toner treatment compositions.
Toners can be used for various applications, including electrographics. Such applications can require toners for printing high quality text and half tone images (see S. Sata et al., xe2x80x9cStudy on the Surface Properties of Polyester Color Toner,xe2x80x9d ISandT NIP13, 149-152 (1997) and Nash, R. and Muller, R. N. xe2x80x9cThe Effect of Toner and Carrier Composition on the Relationship Between Toner Charge to Mass Ratio and Toner Concentration,xe2x80x9d ISandT NIP 13, 112-120 (1997).
In electrographic applications, it is generally desirable to reduce toner particle size in a developer to improve image quality. However, the smaller the toner particles, the more likely that the particles will pack in an electrographic apparatus, such as a copier, and not flow properly, thus adversely affecting the apparatus""s operation and image quality.
One solution can be adding fumed silica as a lubricant to reduce friction and packing, and thus improve the fluidity of the pre-toner particles. However, the use of fumed silica can result in an increase in the negative charge of the treated toner particles. Such a negative charge increase can impede transport of a particle, particularly in multilayer transfer processes in color printers, from one charged surface to another during electrographic operations, e.g., from the developer station to the photoconductor, or from the photoconductor onto the paper.
Alternatively, toner particles can be treated with titanium dioxide. Such formulations are disclosed in U.S. Pat. Nos. 4,943,506, 5,155,000, 5,192,637, 5,429,902, 5,705,303 and 5,747,211. Titanium dioxide treated toners generally have lower charges, and thus are easier to transport through the electrographic apparatus. However, such titanium dioxide treated toners, partly due to their low charge, can create significant airborne dust when replenishing an aged developer, which can include a carrier and a toner, with fresh toner. Such replenishment methods are disclosed in U.S. Pat. Nos. 3,938,992 and 3,944,493.
Attempts have been made to treat toner particles with a combination of silica and titanium dioxide mixtures, as disclosed in U.S. Pat. Nos. 5,272,040, 5,776,646 and 4,623,605, but these efforts have been largely unsuccessful in simultaneously reducing negative toner particle charge and rebuilt airborne dust levels.
The present invention provides a treatment for an electrographic toner. Generally, the treatment includes at least one titanium dioxide particle having a BET surface area of at least about 70 m2/g and/or a diameter no more than about 20 nm, and at least one silica particle. Preferably, the titanium dioxide particle has a BET surface area of at least about 80 m2/g, and more preferably a BET surface area of from about 75 to about 105 m2/g. In addition, the titanium dioxide particle generally has a diameter of no more than about 15 nm, and preferably still, has a diameter of about 7 to about 12 nm. Preferably, the weight ratio of silica particles to titanium particles can be from about 1:0.1 to about 1:2, and more preferably, the weight ratio of silica particles to titanium particles can be from about 1:0.5 to about 1:1.5, and optimally, about 1:1.
The present invention additionally provides an electrographic toner, including at least one toner particle. This particle can include a binder, a pigment and a charge control agent. The toner particle can be treated with the titanium dioxide-silica treatment as discussed above. Preferably, the treatment is applied to the surface of the particle. What is more, the electrographic toner can have a BET surface area to Coulter surface area of 1:1 to 3:1, and 8 micron average particle size. The invention is further directed to a developer including the electrographic toner as discussed above and a carrier. The developer preferably exhibits a charged mass of 10-40 (microCoulomb)/gram, and a rebuilt charge at 10 minutes of less than xe2x88x9245 (microCoulomb)/gram and less than 5 milligrams of generated airborne dust after rebuilding.
Additionally, the present invention provides a method of making a toner. The method includes providing at least one toner particle and treating with at least one titanium dioxide particle having a BET surface area of at least about 70 m2/g and at least one silica particle.
Definitions
xe2x80x9cAirborne dustxe2x80x9d refers to dust that is produced when fresh replenishment toner is mixed with aged developer.
As used herein, the term xe2x80x9celectrographicxe2x80x9d is generally interchangeable with the terms xe2x80x9celectrostatographicxe2x80x9d, xe2x80x9celectrophotographicxe2x80x9d, and xe2x80x9cxerographicxe2x80x9d.
xe2x80x9cCharge characteristicsxe2x80x9d refers to the ratio of charge to mass of a toner in a developer. Preferably, an 8 micron (volume average) particularized toner has a charged mass ratio of 10-50 microCoulomb/gram, and more preferably, 20-45 microCoulomb/gram.
The xe2x80x9cCoulter counterxe2x80x9d measures the number in volume of average particle sizes and the specific surface area of a toner. This measurement is made by suspending the particles in a conductive liquid and monitoring the electrical current between two electrodes emerged in the conductive liquid on either side of a small aperture. The suspension of particles flows through the aperture. Each particle changes the impedance between the electrodes and produces an electric pulse of short duration having a magnitude essentially proportional to the particle volume. The series of pulses are electrically scaled, counted and accumulated in a number of size-related channels, thereby producing a size distribution curve. Assuming the particles are spherical, the Coulter counter also estimates a specific surface area of the toner particles.
The xe2x80x9cBETxe2x80x9d is a measurement of a particles surface area in square meters per gram. This procedure includes degassing a sample overnight under helium gas at 10 psi pressure at room temperature and then subjecting the degassed sample to a flowing mixture of about 70 mol % helium carrier gas and about 30 mol % nitrogen absorbate gas at a pressure of 7 psi and xe2x88x92195 degrees C. The amount of nitrogen adsorbed is used with the BET equation to calculate the surface area. What is more, the ratio of the BET surface area to the Coulter surface area can be used as a measure of the toner shape irregularity.
As used herein, the term xe2x80x9crebuilt chargexe2x80x9d refers to the charge associated with the fresh replenishment toner particles after introducing and mixing with carrier from an aged developer.
Discussion
One embodiment of the invention is a treatment composition for an electrographic toner. The treatment can include at least one titanium dioxide particle having a BET surface area of at least about 70 m2/g and preferably having a BET surface area of from about 75 to about 105 m2/g. What is more, preferably the treatment has at least one titanium dioxide particle having a BET of at least about 80 m2/g. Generally, multiple titanium dioxide particles are used. Furthermore, the titanium dioxide particles used in the present invention preferably have a diameter of not more than about 20 nm, preferably not more than about 15 nm, and more preferably have a diameter ranging from about 7 to about 12 nm. An example of such titanium dioxide particles are those sold under the trade designation MPT313 by Isihara Sangyo Kaisha Ltd of Osaka, Japan.
The treatment can further include a silica particle. Generally, multiple silica particles are used. Preferably, such silica particles have a BET surface area of from about 50- about 350 m2/g and an average particle diameter of about 5-about 40 nm, and more preferably an average particle diameter of about 8-about 20 nm. Examples of such silica particles are those sold under the trade designations R972, RY200 and RY300 from Degussa Corporation, of Akron, Ohio.
Optionally, the silica and/or titanium dioxide particles may be treated with one or more organic or silicone compounds in an amount, preferably, of between about 0.1 to about 20 weight percent of the total weight of silica and/or titanium dioxide particles. Examples of the organic or silicone compounds include silicone oil, polydimethylsiloxane, alkylsilane, and/or alkylsilazane.
The ratio of the weight of silica particles present on the surface of said toner to the weight of titanium dioxide particles on the surface is preferably from about 1:0.1 to about 1:1.5. Preferably, the ratio of the weight of silica particles present on the surface of said toner to the weight of titanium dioxide particles on the surface can range from about 1:0.5 to about 1:1.5. More preferably, the ratio of the weight of silica particles present on the surface of said toner to the weight of titanium dioxide particles on the surface is about 1:1.
Generally, the treatment composition is applied to a toner. The treatment can be applied by mixing, or otherwise coating the toner particles with the treatment composition. In one desired embodiment, the toner is dry blended with the surface treatment composition of the present invention in a high speed lab mixer from 1000-3000 rpm, preferably for 30 to 60 seconds. The silica and titanium dioxide particles can be from about 0.1 to about 10 wt. % of the total toner weight. Preferably, the amount of surface treatment to toner can vary from a weight percent of about 0.1 to about 5, more preferably from a weight percent of about 0.5 to about 3, and optimally from a weight percent of about 1.5 to about 2.5 of the total toner weight.
Preferably, the toner includes a binder, and optionally includes a colorant, a charge control agent, and an anti-blocking agent, which can be blended to form toner particles. Binders can be selected from a wide variety of materials, including condensation polymers such as polyesters as well as both natural and synthetic resins and modified natural resins, as disclosed, for example, in U.S. Pat. No. 4,076,857. Other useful binders can include the crosslinked polymers as disclosed in U.S. Pat. Nos. 3,938,992 and 3,941,898. The crosslinked or noncrosslinked copolymers of styrene or lower alkyl styrenes with acrylic monomers such as alkyl acrylates or methacrylates may also be used. Numerous polymers suitable for use as toner resins are disclosed in U.S. Pat. No. 4,833,060. Consequently, the teachings of U.S. Pat. Nos. 3,938,992, 3,941,898, 4,076,857; and 4,833,060 are hereby incorporated by reference in their entirety. In addition, another desired binder is a bis-phenol based polyester of the acid value between 1 and 40. The toner typically comprises 85 to 95 weight percent by weight of the binder. Such a binder can be propoxylated bisphenol-A combined with furmaric acid.
Optionally, the binder can be compounded with a colorant, i.e., a dye or pigment, either in the form of a pigment flush (a special mixture of pigment press cake and resin well-known to the art) or pigment-resin masterbatch, as well as any other desired addenda known to the art. If a developed image of low opacity is desired, no colorant need be added. Normally, however, a colorant can be included and it can, in principle, be any of the materials mentioned in Colour Index, Vols. I and II, 2nd Edition (1987). Carbon black can be especially useful while other colorants can include pigment blue, pigment red, and pigment yellow. Specific colorants can include copper phthalocyanine, and pigment blue sold under the trade designation LUPRETON BLUE SE1163. The amount of colorant, if used, can vary over a wide range, e.g., from about 1 to about 25, and preferably from about 3 to about 20 weight percent of the toner component. Combinations of colorants may be used as well.
The toner can also contain charge control agents. Particular charge control agents are disclosed in U.S. Pat. Nos. 3,893,935 and 4,206,064, and British Pat. No. 1,501,065, the teachings of which are incorporated herein by reference in their entirety. Furthermore, quaternary ammonium salt charge agents as disclosed in Research Disclosure, No. 21030, Volume 210, October 1981 (published by Industrial Opportunities Ltd., Homewell, Havant, Hampshire, PO9 1EF, United Kingdom) may also be used. Specific charge control agents can include aluminum and/or zinc salts of di-t-butylsalicylic acid. Preferably, the charge control agent is, if used, provided in an amount of about 0.2-about 5% wt. of the total toner weight and preferably in an amount of about 1-about 3 weight percent of total toner weight.
What is more, the toner can optionally contain other additives, such as antiblocking agents and/or waxes, such as polypropylene, polyethylene, or copolymers and blends thereof.
The shape of the toner can be irregular, as in the case of ground toners, or spherical. Spherical particles are obtained by spray-drying a solution of the toner resin in a solvent. Alternatively, spherical particles can be prepared by the polymer bead swelling technique disclosed in European Pat. No. 3905 published Sep. 5, 1979, to J. Ugelstad, as well as by suspension polymerization, such as the method disclosed in U.S. Pat. No. 4,833,060, the teachings of which are incorporated by reference herein in their entirety.
Furthermore, it is desirable that the toner has a ratio of BET surface area to Coulter surface area of about 1:1 to about 5:1, and more preferably has a ratio of about 1:1 to about 3:1, and even more preferably a ratio of about 1:1 to about 1.5:1. Preferably, the median volume average particle size of the toner is in the range of about 2 to about 20 microns, preferably about 7.8 to about 8.5 microns, and a specific surface area of about 0.7-0.8 m2/mL as measure by Coulter.
An exemplary toner formulation is depicted below in Table 1.
In one embodiment of the invention, the electrographic toner polymer particles are prepared by means of an organic solvent/aqueous chemical process, a process frequently referred to as xe2x80x9climited coalescencexe2x80x9d (LC process). In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent by evaporation. The resultant particles are then isolated, washed and dried.
In the practice of this technique, toner particles can be prepared from any type of polymer that is soluble in a solvent and immiscible with water. Thus, the size and distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer, typically silica or latex, and the size to which the solvent-polymer droplets are reduced by agitation.
Limited coalescence techniques of this type have been described in numerous patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of toner particles having a substantially uniform size distribution. Representative limited coalescence processes employed in toner preparation are described in U.S. Pat. Nos. 4,833,060 and 4,965,131 to Nair et al. The method can involve dissolving a polymer material in an organic solvent and optionally a pigment and a charge control agent to form an organic phase, dispersing the organic phase in an aqueous phase comprising a particulate stabilizer and homogenizing the mixture, evaporating the solvent, and washing and drying the resultant product.
If a colorant is used in the toner, the mixture of binder and colorant may be melt compounded and milled to disperse the colorant and other additives in the toner. The mass may be cooled, pulverized into lumps and finely ground. The resulting toner particles can range in diameter from about 0.5 to about 25 xcexcm with an average size of from about 1 to about 16 xcexcm.
Preferably, the toners are combined with a carrier to form a developer. Preferably, the average particle size ratio of carrier to toner particles is from about 15:1 to about 1:1. However, carrier-to-toner average particle size ratios of as high as about 50:1 can be useful. Preferably, the volume average particle size of the carrier particles can range from about 5 to about 50 microns.
U.S. Pat. Nos. 4,546,060 and 4,473,029, the disclosures of which are incorporated herein by reference, describe that the use of xe2x80x9chardxe2x80x9d magnetic materials as carrier particles increases the speed of development dramatically when compared with carrier particles made of xe2x80x9csoftxe2x80x9d magnetic particles. The preferred ferrite materials disclosed in these patents include barium, strontium and lead ferrites having the formula MO6Fe2O3 wherein M is barium, strontium or lead. However, magnetic carriers useful in the invention can include soft ferrites, hard ferrites, magnetites, sponge iron, etc. In addition, the magnetic carrier ferrite particles can be coated with a polymer such as mixtures of Kynar/poly(methyl methacrylate) or silicone resin type polymer. Preferably, the toner is present in an amount of about 2 to about 20 percent by weight of the developer and preferably between 5 and 12 weight percent.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by weight.
The entire disclosures of all applications, patents and publications, cited above or below, are hereby incorporated by reference.